Fish Need Riparian Zones

Fishes Need Riparian Zones

Streamside vegetation removal causes serious problems for fishes and other animals living in the water. Overhanging vegetation and trees shade the stream channel, providing valuable temperature regulation in both summer and winter. In summer, leaves and branches reduce the amount of sunlight striking the stream, preventing an increase in water temperature. Removing riparian vegetation can increase summer water temperatures by up to an average of 3^oC. Warmer water temperatures reduce the oxygen-carrying capacity of the stream, as well as increase the incidence of disease.

Mature riparian vegetation provides streams with a regular source of large organic debris, which benefits fishes by stabilizing streams, forming pools and other habitat features, improving water aeration, and providing fishes with cover and protection from predators. Once the riparian areas have been logged, existing large organic debris is less stable and more likely to move downstream. This process can result in the reduction of pools and the straightening of stream channels.

Probably no group of fishes has had their habitat requirements as well researched and documented as Atlantic and Pacific salmon, fishes which are totally dependent upon clean cool streams with loose gravel substrates. Sediments from erosion can affect salmon and other fishes in numerous ways:

They can destroy spawning habitat by covering the gravels needed to lay their eggs (as used by the pair of spawning coho salmon at right).

Sediments settling out on spawning gravels will smother the eggs, or smother attached periphyton and benthic macroinvertebrates needed by fry for food. One research team found that benthic invertebrate density and biomass was decreased by up to 2/3 after streamside logging.

Suspended sediments also interfere with the ability of salmonids to catch what prey is available, through reducing visibility or forcing changes in movements and behavior.

Spawning coho salmon: Skagit River, WA; Jay DeLong photo

Tennessee shiner (left) and rosyside dace (center) engaged in a multi-species spawning frenzy over a stoneroller's gravel nest. The tail-end of the host stoneroller is visible to the right. These species and many others are intolerant of siltation.

Fish populations are differentially affected by riparian zone removal. As stream waters become warmer, trout and other cold-water species decline in number and biomass. Smaller fishes like minnows and darters disappear as gravel and cobble substrates become silt-laden, denying them feeding and spawning habitat. They are often replaced by more tolerant species, accompanied by a decrease in species diversity.

The threats facing native southestern U.S. fishes are stated in rather alarming terms in Southeastern Fishes Council Regional Reports - 2000 :

"Given the array of threats that are reasonably anticipated, levels of imperilment in southeastern fishes will significantly increase without significant efforts to recover habitats and establish connected river refugia. The cumulative effects of human population growth may push many southeastern fishes to levels of imperilment currently experienced by freshwater mussels. Southeastern fishes have endured previous episodes of persistence-threatening, large scale events (e.g.,pervasive water pollution prior to the Clean Water Act, era of high dam construction). It is remarkable that some species have even persisted this long. However, the rate, magnitude, and permanence of change across the southeastern landscape has never been greater: more change will occur in the next two decades than during the entire history of European colonization. That unprecedented level of pervasive stressors will undoubtedly force marginally persisting fishes closer to extinction."